I hear this a lot. Isn't the issue that iOS patches typically contain security fixes (which in turn telegraph the security flaws in iOS devices for those enterprising enough to look for them). Therefore, if criminals were able to roll the software version of an iOS device back to an older version, they could then in theory take advantage of those known vulnerabilities to further compromise your device?
I kinda get why Apple doesn't permit this. No doubt, there are some people who would be better off if they, say, updated their phone and then regretted it (because of the hit to performance or it changed some things they didn't like) and could subsequently downgrade. But it also seems bad for society as a whole if anybody could do this to any Apple device out there.
Correct me if my take on this issue is incorrect.
That said, I have no idea whether the percentages released by Apple are good or bad. But I do have several friends who are never in any hurry to upgrade, I will give them the heads up whenever Apple releases a new patch, and to each their own, I suppose.